The hypothesis that adenosine is a physiological regulator of coronary flow envisions an exclusively extracellular adenosine pool. Because the amount of adenosine in heart muscle is too large to be compatible with this hypothesis, I obtained evidence that adenosine bound to 2 or more "soluble" cardiac proteins constitutes an intracellular compartment. This project combines physiological and biochemical experiments aimed at: (1) confirming the existence of an intracellular adenosine compartment; (2) obtaining information about how it changes under physiological conditions; (3) describing the properties of the binding proteins; (4) examining their role in cardiac metabolism; and (5) devising techniques to estimate the size of the extracellular adenosine compartment which effects coronary vasomotion.